Everything about Humid totally explained
Humidity is the amount of water vapor in the air. The term "humidity" is usually taken in daily language to refer to
relative humidity. Relative humidity is defined as the amount of
water vapor in a sample of
air compared to the
maximum amount of water vapor the air can hold at any specific
temperature in a form of 0 to 100%. Humidity may also be expressed as absolute humidity and specific humidity. Relative humidity is an important
metric used in
forecasting weather. Humidity indicates the likelihood of
precipitation,
dew, or
fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of
sweating to cool the body by preventing the
evaporation of perspiration from the skin. This effect is calculated in a
heat index table. Warm water vapor has more
thermal energy than cool water vapor and therefore more of it evaporates into warm air than into cold air.
Absolute humidity
Absolute humidity is the quantity of water in a particular volume of air. The most common units are grams per cubic meter, although any mass unit and any volume unit could be used. Pounds per cubic foot is common in the U.S., and occasionally even other units mixing the English and metric systems are used.
If all the water in one
cubic meter of air were condensed into a container, the container could be weighed to determine absolute humidity. The amount of vapor in that cube of air is the absolute humidity of that cubic meter of air. More technically: the mass of water vapor
, per cubic meter of air,
.
»
Humidity during rain
Humidity is a measure of the amount of water vapor dissolved in the air, not including any liquid water or ice falling through the air. For
clouds to form, and
rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is falling from. This normally happens when the air rises and cools. Typically, rain falls into air with less than saturated humidity. Some water from the rain may evaporate into the air as it falls, increasing the humidity, but not necessarily enough to raise the humidity to 100%. It is even possible for rain falling through warm, humid air to be cold enough to lower the air temperature to the
dew point, thus condensing water vapor out of the air. Although that would indeed raise the
relative humidity to 100%, the water lost from the air (as dew) would also
lower the
absolute humidity.
Dew point and frost point
Associated with relative humidity is
dew point (If the dew point is below freezing, it's referred to as the frost point). Dew point is the
temperature at which water vapor saturates from an air mass into liquid or solid usually forming
rain,
snow,
frost, or
dew. Dew point normally occurs when a mass of air has a relative humidity of 100%. This happens in the
atmosphere as a result of cooling through a number of different processes.
Measuring and regulating humidity
There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or
hygrometer. A
humidistat is used to regulate the humidity of a building with a de-humidifier. These can be analogous to a
thermometer and
thermostat for temperature control.
Humidity is also measured on a global scale using remotely placed
satellites. These satellites are able to detect the concentration of water in the
troposphere at altitudes between 4 and 12 kilometers. Satellites that can measure water vapor have sensors that are sensitive to
infrared radiation. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of future
weather forecasts.
Humidity and air density
Humid air is less dense than dry air because a molecule of water weighs less than molecules of
nitrogen and
oxygen.
Isaac Newton discovered this phenomenon and wrote about it in his book
Opticks.
Avogadro's
ideal gas law states that a fixed volume of gas at a given temperature and pressure always contains the same number of molecules regardless of what type of gas it is. Consider a cubic meter of dry air. About 78% of the molecules are nitrogen (N
2), with a molecular weight of 28. Another 21% of the molecules are oxygen (O
2), with a molecular weight of 32. The final 1% is a mixture of other gases. Combining these weights in the correct proportions gives an average molecular weight for air of about 29. If molecules of water vapor (H
2O), of molecular weight 18, replace the diatomic nitrogen or oxygen molecules in this fixed volume then the weight of the air decreases, and hence the density decreases. Thus, humid air has a lower density than dry air at a specified temperature and pressure.
Effects on human body
The human body sheds heat by a combination of evaporation of perspiration,
heat convection in the surrounding air, and
thermal radiation. Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity,
blood brought to the body surface can't shed heat by conduction to the air, and a condition called
hyperpyrexia results. With so much blood going to the external surface of the body, relatively less goes to the active
muscles, the
brain, and other internal
organs.
Physical strength declines and
fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected. This resulting condition is called
heat stroke or
hyperthermia.
Recommendations for comfort
Humans control their body temperature by
sweating and
shivering. The
United States Environmental Protection Agency cites the
ASHRAE Standard 55-1992
Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%, with below 50% preferred to control
dust mites. At high humidity sweating is less
effective so we feel hotter; thus the desire to remove humidity from air with
air conditioning in the summer. In the winter, heating cold outdoor air can decrease indoor relative humidity levels to below 30%, leading to discomfort such as dry skin and excessive thirst.
Effects on electronics
Many electronic devices have humidity specifications, for example, 5 to 95%. At the top end of the range, moisture may increase the conductivity of permeable insulators leading to malfunction. Too low humidity may make materials brittle. A particular danger to electronic items, regardless of the stated operating humidity range, is
condensation. When an electronic item is moved from a cold place (eg garage, car, shed) to a warm humid place (house), condensation may coat circuit boards and other insulators, leading to
short circuit inside the equipment. Such short circuits may cause substantial permanent damage if the equipment is powered on before the condensation has
evaporated. A similar condensation effect can often be observed when a person wearing glasses comes in from the cold. It is advisable to allow electronic equipment to acclimatise for several hours, after being brought in from the cold, before powering on. The inverse is also true.
Excessively high humidity causes corrosion in electronics. Low humidity causes static electricity and spontaneous shutdown of servers in data centres. Electronics, and more specific TTL technology, can't handle voltages that exceed the supply voltage by a small margin before it'll blow and cause it to malfunction. Therefore humidity is an important measure in the control of data centre facilities.
Most humid places on earth
The most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in South and Southeast Asia seem to be among the most humid.
Kolkata, India;
Kerala, India; and
Bangkok, Thailand experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm Sauna. Darwin, Australia experiences an extremely humid wet season from December to April.
Kuala Lumpur and
Singapore have very high humidity all year round because of their proximity to water bodies and the
Equator and overcast weather; despite sunshine, perfectly clear days are rare in these locations and it's often misty. In cooler places such as Northern Tasmania, Australia high humidity is experienced all year due to the ocean between mainland Australia and Tasmania. In the summer the hot dry air is absorbed by this ocean and the temperature rarely climbs above 30 degrees Celsius.
In the United States the most humid cities, strictly in terms of
relative humidity, are
Forks and
Olympia, Washington. This fact may come as a surprise to many, as the climate in this region rarely exhibits the discomfort usually associated with high humidity. Dew points are typically much lower on the West Coast than on the East. Because high dew points play a more significant role than relative humidity in the discomfort created during humid days, the air in these western cities usually doesn't feel "humid."
The highest dew points are found in coastal
Florida and
Texas. When comparing Key West and Houston, two of the most humid cities from those states, coastal Florida seems to have the higher dew points on average. But, as noted by Jack Williams of USA Today, Houston lacks the coastal breeze present in Key West.
Further Information
Get more info on 'Humid'.
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